During mining operations in an underground gassy mine comprising a non-combustible ore (specifically trona), methane is often liberated from methane-bearing strata.
Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). A vast deposit of the mineral trona is found in southwestern Wyoming near Green River. This deposit includes layers of trona and mixed trona and halite (rock salt or NaCl) which covers approximately 2,600 km2. The major trona beds range in size from less than 428 km2 to at least 1,870 km2. By conservative estimates, these major trona beds contain about 75 billion metric tons of ore. The different beds overlap each other and are separated by layers of shale and marlstone. The quality of the trona varies depending on its particular location in the stratum.
A typical analysis of the trona ore mined in Green River is as follows:
TABLE 1ConstituentWeight PercentNa2CO343.2NaHCO333.7H2O (crystalline and free moisture)15.6NaCl0.1Insolubles7.3
The sodium sesquicarbonate found in trona ore is a complex salt that is soluble in water. The mined trona ore is processed generally in a surface refinery to remove the insoluble material, organic matter and other impurities to recover the valuable alkali contained in the trona.
The most valuable alkali produced from trona is sodium carbonate. Sodium carbonate is one of the largest volume alkaline commodities produced in the United States. In 2007, trona-based sodium carbonate from Wyoming comprised about 91% of total U.S. soda ash production. Sodium carbonate finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
The trona deposits found in Southwestern Wyoming are formed in multiple beds in the Wilkins Peak Member of the Eocene Green River Formation at depths ranging from 240 to 910 meters (800-3000 feet). The Wyoming trona deposits are evaporites that form substantially horizontal beds. The beds vary greatly in thickness, from about 0.3 meter to about 5 meters (about 1-16 feet). The underground formation containing the trona beds generally includes multiple methane-bearing layers as well. For example, layers of mainly weak, laminated green-grey shales and oil shale may be found in strata both above and below a trona bed. Both overlying and underlying shale layers can liberate methane during mining. It is also possible for marlstone layers to liberate entrapped methane upon fracture. The trona itself contains very little carbonaceous material and therefore liberates very little methane. Yet methane is liberated during mining due to its release from the surrounding shale layers
The mining techniques employed may include longwall mining, shortwall mining, solution, mining, room-and-pillar mining, or various combinations.
When utilizing a trona mining technique that either exposes or fractures one or more methane bearing shales, a significant amount of mine gas can be liberated from the fractured oil shale(s). Mining techniques, such as longwall mining, that involve overburden caving may release substantial volumes of methane. Mine methane is defined as methane gas liberated as a direct consequence of mining activity. Along with the mine methane lower concentrations of other gases such as non-methane hydrocarbons, nitrogen, ammonia, and carbon dioxide, may be found.
Once released, mine methane can mix with the mine ventilation air. In such event, the released methane must be quickly diluted with fresh air to safe levels well below the methane lower explosive limit (LEL), which in normal air is 5% methane. There is indeed an explosive range for methane in air from 5 to 15%. In most cases this ventilation air methane (VAM) must be diluted to below 1% in the return airways of the mine's ventilation system in order to meet legal requirements. This high dilution in ventilation air must be done to ensure the safety of mine personnel, regardless of the mining technique employed.
By using the mine ventilation system, the methane concentration is diluted with air from a high level to a low level. The greater the volumetric flow rate of released methane, the greater the volumetric flow of ventilation air required to dilute it. If there is an increase in methane release, additional air flow is necessary for dilution to achieve a methane content generally well below the LEL. Additionally, safety regulations require a maximum allowable methane content in the return airways where personnel is present; this maximum allowable methane content is generally 1% or less. In the case of a bleeder system, the methane content may be as high as 2%.
The amount of methane released by specific mining techniques can vary widely, but in general, techniques that expose and/or fracture the methane-bearing strata will release significantly larger volumes of mine methane than those that do not. The additional air requirement for mining techniques which result in higher methane release will also increase ventilation pressure which results in increased air leakage through ventilation structures and increased energy consumption.
In some mines, techniques such as longwall mining can release so much gas that they require an additional drainage system to directly extract methane from drainage wells drilled above or in a ‘gob’ (also called ‘goaf’), an area of fractured rock that forms as the mine roof collapses following ore extraction. Direct methane drainage reduces the amount of gas in the gob and surrounding rock, and therefore less methane is available to be released into the mine airways. This allows for a reduced ventilation air flow, but does not eliminate the need for air ventilation.
While mine ventilation systems are effective in ensuring safe working conditions underground, they create an environmental problem at the surface, as the ventilation air methane (VAM) is generally exhausted to the atmosphere. Even though a drainage system can be effective in recovering gob gas with high methane content, the drainage gas flow rate to the surface is typically at least one or two orders of magnitude lower than the flow rate of return ventilation air. Thus there may be a much higher volumetric output of methane exhausted to the surface from ventilation air than from a gob vent well.
Because ventilation air flow rates are so high, ventilation air methane can constitute a large source of methane emissions from gassy mines contributing significantly to global greenhouse emissions. As a greenhouse gas, methane is 21 times more effective in heat trapping than carbon dioxide over a 100-year period. Thus, efforts in mitigating methane emissions from ventilation air can provide significant environmental benefits. The global carbon market now offers an incentive for mitigating these emissions in the form of carbon credits that constitute an additional revenue stream for gassy mines. Mitigation thus provides some value to the mine operators.
However other significant energy and economic benefits could be obtained if, in addition, the energetic value of the methane (calorific value of 1000 Btu per cubic feet) could be captured. VAM exhaust essentially wastes a potential clean energy resource, but it is difficult to use as an energy source because of the large air volume and low methane content. Flow rates of several hundred thousand to several millions of actual cubic feet per minute or ACFM of ventilation air containing less than 1% methane are typical. The low concentration of methane requires either use of the ventilation air in its dilute state or concentration up to levels that can be used in methane-fueled engines. Concentration can be costly and difficult to achieve, and utilization has been focused on oxidation of very low concentration methane.
For oxidation even at such low CH4 levels, one may employ processes classified as thermal and catalytic oxidizers. A thermal oxidizer for example is a very large, expensive, complex, and inefficient device which can be operated with levels around 1-1.5% methane. But since the ventilation air with a small amount of methane is not easily combustible, such operation requires significant pre-heating which renders a thermal oxidizer quite costly to operate. In most cases due to size and cost constraints, the thermal oxidizer can only treat a fraction of the total ventilation air exhaust with the remainder going to the atmosphere. The main limitation of the thermal and catalytic oxidizers is the difficulty in extracting useful energy for power generation from VAM, so these oxidizers generally are only used to mitigate the greenhouse impact of the treated methane emissions. Any beneficial utilization of this ventilation air methane presents significant challenges. In fact, the only practical applications for the use of VAM as an energy source would be as combustion air for methane-fueled devices located in close proximity to the return air exhausts.
Unlike coal mining, Applicants believe that trona mining is particularly well suited for beneficial utilization of this ventilation air methane. It presents an unusual combination of both having gassy trona mines that emit substantial methane during mining and nearby surface processes that can use the energetic value of the emitted methane. Indeed a trona mine has some level of ventilation air methane near a soda ash refinery where the trona is processed, and this soda ash refinery near the trona mine will have surfaces appliances that can combust ventilation air methane.
Although these foregoing issues have been and will be described in terms of trona mining, they also apply to any mine from which a non-combustible ore (e.g., evaporite or metal/non-metal ore) is extracted and which is capable of liberating methane during the mining of the non-combustible ore.